Tow Vehicle Charging while in route

Hi all,

I am wondering about how to handle my travel trailer batteries while in tow. When home I use a small charger/maintainer to keep the TT batteries (2 6v deep cycle) topped off. When I get to camping, I plug in my Renogy solar suitcase to get solar charging. its only 100w so depending on how long I camp and how much I use while doing so, the batteries can get run down. The standard 7pin connection from my vehicle "charges" the batteries while traveling. The TT batteries seems to do fine however I believe its ruining my truck battery, since once the low battery from the trailer is seen by the trucks alternator it boosts up the voltage to charge. I believe it's over charging my truck battery due to the lower voltage of the trailer batteries. So what do you all do? somehow disconnect the trailer batteries from the vehicle? its not straight forward, as the trailer batteries are required to be connected for at least the emergency brake-away system and to run the refrigerator etc. while in transit. Eventually I'll get more panels to keep the batteries charged but I assume this is a problem for others as well. I am going to be converting over to a LifePo4 battery in the near future so it will be a question on how to maintain it then also.

Thanks!

Thats something I have thought about over the years towing, but in reality it's never been an issue. I have been running 2 GC batteries for at least 20 years and regular "deep cycle" for 20 years before that. My last oem towing battery lasted 8 years-and thats with at least 60k km towing, sometimes with a very depleted trailer battery. One thing I haven't done for the longest time is leave the fridge on 12v. I just leave it off. The newer trailer doesn't even have that option...John

Thanks for the reply John. I have a 2017 tundra and my truck battery appears to on its last leg. It's possible that it's coincidence and the next one will last however I generally get at least 5/6 years from my vehicle batteries, and I am noticing some wetness on the top of the truck battery. What do you mean about leaving your fridge on 12v? Both my trailers have had a gas option or an AC power "auto" option. Gas still requires 12v to keep it running however its a small amount. I assume some can run on 12v exclusively. While traveling long distances you don't run your refrigerator? Anyway thanks for the info...

No, I don't run my fridge while traveling, I won't leave the propane on. My older trailers and camper had the option to run on 12v only,(it's just a 12v heating element)-no propane, that's what I was talking about. Thats pretty bad life for a battery, 2010 F150 and I put in a new battery 1 1/2 years ago, my brothers Dodge Ram (poor guy) battery lasted 11 years and he tows as well. The amount of 12v to run the fridge on propane is very little.

Here in Australia the general concensus is with 3 way fridges is to run a seperate heavy gauge wire to run the fridge while in transit , with a relay to shut off when the vehicle is turned off so as to not drain the starter battery
Another heavy gauge seperate cable run is for the charging of the trailer batteries, usually incorporating a dc dc unit near the trailer batteries to regulate amps drawn from the alternator and charging profiles to suit batteries , though lm not sure the ratio of people who bother to use a dc dc or not ...... either way l can't see much harm coming to your vehicle battery as typically lead batteries like staying full and l haven't heard this being a problem on the forums over here ....ever
Out of interest what voltage are you seeing ?

I need to check it next time I run the trailers batteries get low. I can't figure out how why else my truck battery would be going bad after 3 1/3 years. Maybe it's just a crap battery.

vdubnut said:

I need to check it next time I run the trailers batteries get low. I can't figure out how why else my truck battery would be going bad after 3 1/3 years. Maybe it's just a crap battery.

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Also my alternator generally stays right about 13.5 - 14 volts while running.

vdubnut said:

I need to check it next time I run the trailers batteries get low. I can't figure out how why else my truck battery would be going bad after 3 1/3 years. Maybe it's just a crap battery.

Click to expand...

The quality of OEM batteries isn't very good these days. Three years on a battery that hasn't had some TLC is about right. I had one of my OEM Ford Motorcraft batteries replaced under warranty because it was leaking. I keep the two (parallel) batteries on a smart charger most of the time. The truck will spend weeks without getting started.

A good DC-DC converter/charger is the way to go. Even with SLA's it's helpful.

On my rig, (2012 2500HD diesel and 5th wheel) I just added a Victron 12/24 DC-DC converter to directly charge my 24v solar bank.
You'll need to size the voltage output to whatever you want.
Note - I've put my breakaway system on a dedicated SLA with a breakaway charge controller. This also feeds from the Tow vehicle 12v.

Now, interestingly, by adding a smart controller, I found out that I need to clean up my trailer connector - when charging starts, voltage at my hitch is dropping to 8.5v. I NEVER would have found that if I hadn't sprung for a controller like the victron with the built-in bluetooth/app reporting.

Sounds like you need a DC-DC charger in your trailer. Also will be essential when/if you upgrade to LifePo4.
by the time the voltage drop gets down to the batteries in the trailer, there's not enough voltage to charger to a decent SOC.
The vehicles battery is probably getting killed by equalizing to the low state of charge in the trailers battery when the engines not running.

vdubnut said:

The standard 7pin connection from my vehicle "charges" the batteries while traveling. The TT batteries seems to do fine however I believe its ruining my truck battery, since once the low battery from the trailer is seen by the trucks alternator it boosts up the voltage to charge. I believe it's over charging my truck battery due to the lower voltage of the trailer batteries. So what do you all do?

Click to expand...

I wonder if this is actually the case? Is the 12V charge line from the TV 7pin to the TT indeed being monitored by the TV? IF the TT battery is quite low, and the TV battery is full, does the TV actually see both batteries as 1? In which case, yea, it'll assume the TV battery still needs charging, but in reality, it's doesn't. Its just averaging both, so your suspicion that the TV battery is getting overcharged is, in my opinion, valid. Or.......
Is that 12v charge line from the 7pin just an independent 12v source that just delivers power all the time, regardless of the TV batteries state? Kind of like a 12v accessory line? In which case the TT batteries better have some protection, otherwise, on a long trip, getting 12/13/ or 14V could damage the TT batteries if they fill up while driving.
I'm just thinking out loud. I'm still trying to figure out how that part of the 7pin functions.

A DC-DC charge controller is the way to go. It converts the charge source to the correct voltage for the destination battery.

On DC the typical fridge heater element pulls ~250W or 20A. Most vehicles can handle that load on the alternator while moving but it will drain a typical 12V starter battery quickly if you forget to disconnect when the TV is parked. One solution is a battery isolator relay that cuts off power to the tow connector if the TV battery drops below a useful voltage.